cyclegraph: detect cycles immediately

cyclegraph: detect cycles immediately

Previously, we have used a special case in CycleGraph to delay detection
of a mo-cycle (actually, an RMW atomicity violation) long enough so that
a RMW can "reorder" across another RMW by passing its future value.
Unfortunately, this only works some of the time, as the execution
*may or may not* be a feasible prefix which allows the sending of the
future value.

Instead, we need to pull this special case out of CycleGraph and (in
subsequent commits) will add a more proper solution in model.cc.